Method of using long cellized codes in a joint detection system

ABSTRACT

The invention discloses a method of applying a long cell-code in a joint detection system. At transmitters, multiples of the spreading code (spreading factor) is taken as the length of the long cell-code to scramble the signal, and at receivers, the method still takes the multiuser detection to process the received signal. The method includes that: for every antenna unit, making channel estimation to obtain a channel estimation result of each antenna unit; generating a first mid-matrix of the received data of each antenna unit that relates to selected length of said long cell-code and the channel estimation result; based on the first mid-matrix, generating a second mid-matrix and its associate matrix, and then based on said generated second mid-matrix and its associate matrix, generating a third mid-matrix; making Cholesky decomposition of said third mid-matrix, wherein the number of decomposition order relates to the length of said long cell-code; making demodulation processing based on said Cholesky decomposition result and said received signals of all antenna units having been matching filtered. The whole computation loads of the method are acceptable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2003/000961 filed on Nov. 13, 2003. This application claims thebenefit of Chinese Application No. 02148622.0 filed on Nov. 13, 2002.The disclosures of the above applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a CDMA cellular mobile communicationsystem where training sequence is applied, more especially, to ademodulation method at receivers where multiuser detection is deployedwhile long scrambling code is applied at transmitters.

BACKGROUND OF THE INVENTION

One of the goals for any cellular mobile communication system is to makeuse of radio resources as much as possible and to provide more andbetter services. Multiuser detection can obviously raise systemperformance and capacity, and comparing with the convention RAKEreceiver, the spectrum efficiency is almost double.

Nevertheless, computation loads of the multiuser detection are huge andin direct proportion to square of the number of subscribers. It isimpossible for the present microprocessor and Field Programmable LogicArray (FPLA) to perform such computation loads. Therefore, in 3GPP twosystems: the wide-bandwidth time division duplex (WB-TDD) andnarrow-bandwidth time division duplex (NB-TDD), where the multiuserdetection is definitely used, the maximum length of spreading code (orspreading factor) in a time slot is 16, thereby the maximum number ofsubscribers is 16, and the length of scrambling code of local cell, i.e.cell-code for cell identification is 16 too.

The main purposes of the cell-code specified in the CDMA TDD standardare to counteract interference from adjacent cells and to whiten thesignals from adjacent cells. This cell-code is different with the longcode and short code used in other CDMA systems. In those CDMA systems,all subscribers share one long code and one short code, subscribers aredifferentiated by different phases of the long code and cells aredifferentiated by different phases of the short code. However, each cellhas its own cell-code. For example, the NB-TDD system has 128cell-codes.

Since the length of the spreading code and the cell-code of cells areall 16, the length of the spreading modulation code generated by them is16 too. When there are 128 cell-codes in a system (the system cansupport 128 cells simultaneously), and each cell-code makes dot producewith 16 WALSH codes, then the system generates 16×128=2048 spreadingmodulation codes (a spreading modulation code is made by dot produce theWALSH code and the cell-code). In other words, there are 2048 spreadingmodulation codes with length 16. It is very difficult to guarantee thatthere are not only 128 groups of orthogonal codes in 2048 spreadingmodulation codes but also better cross-correlation of different codegroups.

Selection of the cell-code length should be taken into account; iflength of the cell-code is too short, it is a disadvantage to counteractinterference of adjacent cells and is impossible to whiten signals.Taking the NB-TDD system as an example, there are 128 cell-codes, and itis very difficult to have good cross-correlation between these codegroups. At present, although from the statistical point of view thecross-correlation properties between codes are better, but correlationbetween some code groups is very high or even completely correlative.For example, in NB-TDD, the first code and the 126^(th) code are asfollows: Code 1 1 1 1 1 1 −1 1 −1 1 −1 −1 1 1 1 −1 −1 Code 126 1 1 1 1−1 1 −1 1 −1 1 1 −1 1 1 −1 −1

When Code 1 makes dot product with WALSH 12, the spreading modulationcode is:

1 1 1 1 −1 1 −1 1 −1 1 1 −1 1 1 −1 −1.

When Code 126 makes dot product with WALSH 0, the spreading modulationcode is:

1 1 1 1 −1 1 −1 1 −1 1 1 −1 1 1 −1 −1.

It is seen that these two spreading modulation codes are identical, andthis is called repetition codes in the system.

As viewed from above instance, with correlation between some codes beingvery high or even completely correlative, there are some repetitioncodes in 2048 spreading modulation codes. The repetition code in a CDMAsystem is a disaster, especially when two subscribers at adjacent cellsare allocated with the repetition code. In this case, these twodifferent subscribers have the same spreading modulation code withlength 16; only their midamble codes are different. Although these twosubscribers can be differentiated by the midamble code, when thesubscriber signals come from the same direction (signals from anydirection in case of a receiver with omni-antenna), their strong pathsare basically coincidence during demodulation, so it is impossible todifferentiate the subscribers with the midamble code. With the smartantenna and the code allocation algorithm, the interference can besuppressed in certain degree, but it cannot be completely eliminated.Once different subscriber signals are spread and modulated with the samespreading modulation code, and they arrive the demodulation end at thesame time, then strong interference will appear at the receiver. It isalmost definitely to say that in this case the receiver cannot correctlydemodulate the received signals so that the spreading gain disappearsand the system cannot work normally at an identical frequency.

Furthermore, amplitude-frequency characteristic in the band of signalsis worse, and cannot satisfy the whitening requirement, so thedemodulation is difficult.

There are three solution for the above problem: first solution, tochange the scrambling code i.e. cell-code to make that there is norepetition code in the 2048 spreading modulation codes; second solution,with the smart antenna and the code allocation algorithm, to guaranteethat no repetition code is used simultaneously at adjacent cells; thirdsolution, to keep longest length of the spreading code being 16 and themaximum number of subscribers being 16, but a long cell-code, such aslength of 32, 64 or 128 (a multiple of 16) using for differentiatingsignals from different cells.

With the first solution, it is necessary to look for new cell-codes, buteven though these scrambling codes can be found, it is difficult tochange the connatural cross-correlation between codes (because somecodes are not completely correlation, but they have high correlation).

With the second solution, the repetition code problem can be avoided incertain degree, but cannot be eliminated completely, and the impact ofhigh correlation codes cannot be avoided.

With the third solution, the repetition code problem is mostly overcome,and the signals are whitened with the long cell-code; this makes thatspectrum of the modulated spreading signals becomes more flat, and thepeak-average power ratio is decreased, so that requirement to filterperformance is reduced and radio frequency system performance is raised.Nevertheless, with this solution, signal-processing method needs to bechanged and the computation loads are increased.

When the long cell-code is applied at the transmitter and jointdetection is still made at the receiver, the computation loads will bemuch more increased. Up till now, there is no any disclosed method toimplement this solution.

SUMMARY OF THE INVENTION

Objective of the invention is to design a method that uses longcell-code in the joint detection system, which is one of above thirdsolutions. The method takes a long cell-code, which is a multiple of thesystem maximum spreading factor 16, such as 32, 64 or 128 etc., toscramble the transmitted signal; and at the same time, makes themultiuser detection to process the received signal. In this way, thecomputation loads are acceptable.

Objective of the invention is implemented in the following scheme:

A method for applying a long cell-code in a joint detection system,comprising, taking the long cell-code of which length is multiples ofspreading factor as a scrambling code of a local cell. Wherein thelength of the long cell-code is multiples of 16.

A demodulation method in a joint detection system where a long cell-codeis applied, comprises,

A. making a channel estimation for midamble code part of a signalreceived by every antenna unit, to obtain a channel estimation result ofeach antenna unit;

B. calculating a first mid-matrix of the received signal of each antennaunit; wherein the first mid-matrix relates to length of the longcell-code and the channel estimation result of each antenna unit, lengthof the long cell-code is multiples of 16;

C. based on the first mid-matrix of the received signal of each antennaunit, calculating a second mid-matrix and its associate matrix; and thenbased on the second mid-matrix and its associate matrix, calculating athird mid-matrix, which is a symmetric definite matrix;

D. making Cholesky decomposition for the third mid-matrix, wherein thenumber of decomposition order relates to the length of long cell-code;

E. matched filtering the received signals of all antenna units, and thenmaking demodulation calculation based on the Cholesky decompositionresult of the third mid-matrix and the matching filtered receivedsignals.

In this invention, in adjacent cells the length of the cell-code is themultiple of 16, but the spreading code length keeps 16, so the frameformat is unchanged, the repetition code is avoided, and theinterference is obviously suppressed. At the same time, the receiverstill applies multiuser detection, so the demodulation performance iskept, the cell signal is more whitened, the modulated spreading signalis more flat, the peak-average power ratio is decreased, and therequirement of the filter performance is reduced.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

A processing method for the long cell-code signal at the receiver, willbe described in more detail in the following and six parts arecomprised.

b 1. The receiver receives signals and makes channel estimation for allchannels. The channel estimation result h^(k) ^(a) of the k_(a) ^(th)antenna unit is calculated with the following formula:h ^((k) ^(a) ₎ =IDFT( G ⁻¹ ·DFT( e _(m) ^((k) ^(a) ₎ )), k _(a)=0 . . .K _(a)−1,wherein

-   a symbol with underscore denotes a vector (it is the same    hereinafter);-   DFT means a discrete Fourier transformation;-   IDFT means an inverse discrete Fourier transformation;-   e_(m) is the midamble code of the received data;-   K_(a) is the total number of the antenna units;-   k_(a) is one of the antenna units;-   G ⁻¹ is the inverse matrix of the correlation matrix of a midamble    code;-   h is channel estimation result, and there are K_(a) channel    estimation results in total.

2. According to the K_(a) channel estimation results and the selectedcell-code L, a second mid-matrix, called A matrix, is generated in foursteps:

First step, the long cell-code L is divided by 16 to obtain M=L/16sections; each section makes dot product with the WALSH code,respectively, and then multiplies with a corresponding value j (j is avalue for angle transformation based on communication standard) toobtain M vectors c _(m) , the length of c _(m) is 16, and m is from 1 toM;

Second step, Compute every matrix with (16+(window length−1))×16 in thefirst mid-matrix A^(ka) according to the following formula (said windowlength is the window length during channel estimation):b _(m) ^((k) ^(a,) _(k) ^(vru)) =h ^((k) ^(a,) _(k) ^(vru)) {circle over(×)} c _(m) ^((k) ^(vru)) , k _(vru)=0 . . . K _(vru)−1, k _(a)=0 . . .K _(a)−1wherein:

-   b _(m) is a column of the matrix;-   k_(a) is one of the K_(a) antenna units and takes value from 0 to    K_(a)−1;-   K_(vru) is the total number of code channels occupied by the    subscriber;-   k_(vru) is one of the K_(vru) code channels and takes value from 0    to K_(vru)−1;-   {circle over (×)} is the convolution operation.

Third step, Based on the M matrixes obtained at Step 2, the matrixesB^(ka) ₁, B^(ka) ₂, . . . B^(ka) _(M) of the first mid-matrix A_(ka),shown in the following, are generated and then repeated (in thefollowing first mid-matrix A_(ka), the line rectangle, sloping linerectangle and point rectangle represent the B^(ka) ₁, B^(ka) ₂ andB^(ka) _(M), respectively, to show the repeat relationship). The numberof the unrepeated matrixes B^(ka) _(m) in the first mid-matrix A^(ka) isrelated with the c _(m) , i.e. the cell-code length. The length of thelong cell-code is multiple of 16, and the length of c _(m) is 16. Exceptthe B^(ka) ₁, B^(ka) ₂, . . . B^(ka) _(M), other positions in the firstmid-matrix A^(ka) are zero, and the computation loads in the successivesteps will be greatly decreased.

Fourth step, compute the second mid-matrix A based on the K_(a) computedresults in the third step. $A = \begin{bmatrix}A^{k_{1}} \\A^{k_{2}} \\A^{k_{3}} \\\vdots \\A^{k_{a}}\end{bmatrix}$

3. Based on the second mid-matrix A, a third mid-matrix called R matrixwill be generated in two steps:

First step, Compute the associate matrix A′ of the matrix A;

Second step, Compute the matrix R, wherein R=A′ A, according to thefollowing formula: $R = \begin{bmatrix}{R0}_{1} & {R1}_{1} & \quad & \quad & \quad & \quad & \quad \\{R1}_{1} & {R0}_{2} & {R1}_{1} & \quad & \quad & \quad & \quad \\\quad & {R1}_{2} & {R0}_{3} & \quad & \quad & \quad & \quad \\\quad & \quad & \quad & \quad & \quad & \quad & \quad \\\quad & \quad & \quad & \quad & {R0}_{M} & {R1}_{M} & \quad \\\quad & \quad & \quad & \quad & {R1}_{M} & {R0}_{1} & {R1}_{1} \\\quad & \quad & \quad & \quad & \quad & {R1}_{1} & \quad\end{bmatrix}$

The above R matrix is a symmetric definite matrix. The characteristicsof the matrix R are that: the R0₁, R0₂, R0₃ . . . RO_(M) are distributedand repeated in the diagonal of the matrix, and the R1₁, R1₂, . . .R1_(M) are distributed and repeated symmetrically in both side of thediagonal. The matrix R is used for Cholesky decomposition in the nextstep.

4. The matrix R is decomposed based on the usual Cholesky decompositionformula R=H^(T)H (T is the transpose symbol). When the long cell-codelength is 48, 64, 128 . . . , i.e. three multiples or more than threemultiples of 16, the decomposition result is a sparse triangle matrix Hshown in the following: $H = \begin{bmatrix}H_{1} & H_{2} & \quad & \quad & \quad & \quad & \quad & \quad & \quad \\\quad & H_{3} & H_{4} & \quad & \quad & \quad & \quad & \quad & \quad \\\quad & \quad & H_{5} & H_{6} & \quad & \quad & \quad & \quad & \quad \\\quad & \quad & \quad & \quad & \quad & \quad & \quad & \quad & \quad \\\quad & \quad & \quad & \quad & \quad & H_{41} & \quad & H_{42} & \quad \\\quad & \quad & \quad & \quad & \quad & \quad & H_{43} & \quad & H_{44}\end{bmatrix}$

Wherein H₁, H₃, H₅, . . . H₄₃ are the 16×16 triangle matrixes, and H₂,H₄, H₆, . . . , H₄₄ are the 16×16 square matrixes, all other elementsare zero.

5. The received signal is matching filtered with the following formula:e _(MF) =A′×e;wherein:

-   e is the received signal of all antenna units;-   e _(MF) is the received signal of all antenna units except the    midamble code;

A′ is the associate matrix of the matrix A.

6. Based on the obtained e _(MF) and matrix H, the demodulationprocessing is done in a conventional way which is solution to aequitation, and the receiver will perform the received signal processingwith the long cell-code based on the multiuser detection.

With the above computation procedure, when the spreading factor is still16, and the antenna array is a signal antenna and eight antennas, thecomputation loads of applying the long cell-codes, such as 32, 64 and128, are shown in the following table, wherein the MOPS is ten thousandtimes of operation per second, for example 80MOPS is eighty thousandtimes per second. Cell-code length Single antenna Eight antennas 16 80MOPS 283MOPS 32 130MOPS 400MOPS 64 192MOPS 595MOPS 128 229MOPS900MOPS

The above table shows that when the spreading factor is still 16 and thelonger cell-code is used, such as 32, 64 or 128, not only theinterference between cells is suppressed effectively, but also thesystem frame format need not be changed, and the computation loads areacceptable. When the scrambling code length 32 is applied, thecomputation loads are larger about 50% compared with the scrambling codelength 16. Even though the scrambling code length 128 is applied, thecomputation loads only increase about three times; this is acceptable.

Without the computation method of the invention, when a long cell-codeis applied, such as 32, 64 or 128, it is necessary to compute directlythe matrix A with 32, 64 or 128 dimension, respectively, and to computethe associate matrix A′ of the matrix A and to decompose these matrixes;the computation loads are directly proportional to three cube of thematrix dimension. It is impossible for the present computation device toperform it, and this is why the cell-code length with 16 is applied inthe TD-SCDMA mobile communication standard. With the inventiondemodulation method, the original large matrix is decomposed intoseveral matrixes with (16+(window−1))×16 (the B^(ka) ₁, B^(ka) ₂, . . ., B^(ka) _(M) of the first mid-matrix), so the computation loads areacceptable.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A method for applying a long cell-code in a joint detection system, comprising, taking the long cell-code of which length is multiples of spreading factor as a scrambling code of a local cell.
 2. The method according to the claim 1, wherein the length of the long cell-code is multiples of
 16. 3. A demodulation method in a joint detection system where a long cell-code is applied, comprising, A. making a channel estimation for midamble code part of a signal received by every antenna unit, to obtain a channel estimation result of each antenna unit; B. calculating a first mid-matrix of the received signal of each antenna unit; wherein the first mid-matrix relates to length of the long cell-code and the channel estimation result of each antenna unit, length of the long cell-code is multiples of 16; C. based on the first mid-matrix of the received signal of each antenna unit, calculating a second mid-matrix and its associate matrix; and then based on the second mid-matrix and its associate matrix, calculating a third mid-matrix, which is a symmetric definite matrix; D. making Cholesky decomposition for the third mid-matrix, wherein the number of decomposition order relates to the length of long cell-code; E. matched filtering the received signals of all antenna units, and then making demodulation calculation based on the Cholesky decomposition result of the third mid-matrix and the matching filtered received signals.
 4. The method according to the claim 3, wherein Step A comprises, calculating channel estimation result h^(k) ^(a) of the k_(a) ^(th) th antenna unit with the following formula: h ^((k) ^(a) ₎ =IDFT( G ⁻¹ ·DFT( e _(m) ^((k) ^(a) ₎ )), k _(a)=0 . . . K _(a)−1, where e_(m) is the midamble code of the received signal; k_(a) is one of the total K_(a) antenna units and takes value 0 to K_(a)−1; G ⁻¹ is the inverse matrix of the correlation matrix of a midamble code; and h is a channel estimation result; Step B comprises, B1. dividing the long cell-code L by 16 to obtain M=L/16 sections; making dot product with the WALSH code for each section, respectively, and then multiplying with a corresponding value j, which is a angle transform based on communication standard, to obtain M vectors c _(m) , the length of c _(m) is 16, and m is from 1 to M; B2. computing each column of a matrix in the first mid-matrix A^(Ka) with the formula b _(m) ^((k) ^(a,) _(k) ^(vru)) =h ^((k) ^(a,) _(k) ^(vru)) {circle over (×)}c _(m) ^((k) ^(vru)) , to obtain M matrixes B^(ka) ₁, B^(ka) ₂ . . . B^(ka) _(M); forming the first mid-matrix with repeating the obtained M matrixes in a diagonal direction of the first mid-matrix, wherein k_(vru) is one of the total K_(vru) code channels occupied by the subscriber and takes value from 0 to K_(vru)−1, and, each of matrixes is constituted of M columns; of matrixes is constituted of M columns, as shown in the diagram in the specification. Step C comprises, C1. calculating the second mid-matrix A according to K_(a) results of first mid-matrix, and then calculating the associate matrix A′ of the second mid-matrix A: ${A = \begin{bmatrix} A^{k_{1}} \\ A^{k_{2}} \\ A^{k_{3}} \\ \vdots \\ A^{k_{a}} \end{bmatrix}};$ C2. Obtaining the third mid-matrix R from the formula R=A′ A, ${R = \begin{bmatrix} {R0}_{1} & {R1}_{1} & \quad & \quad & \quad & \quad & \quad \\ {R1}_{1} & {R0}_{2} & {R1}_{1} & \quad & \quad & \quad & \quad \\ \quad & {R1}_{2} & {R0}_{3} & \quad & \quad & \quad & \quad \\ \quad & \quad & \quad & \quad & \quad & \quad & \quad \\ \quad & \quad & \quad & \quad & {R0}_{M} & {R1}_{M} & \quad \\ \quad & \quad & \quad & \quad & {R1}_{M} & {R0}_{1} & {R1}_{1} \\ \quad & \quad & \quad & \quad & \quad & {R1}_{1} & \quad \end{bmatrix}};$ Step D comprises, making Cholesky decomposition according to the formula: R=H^(T)H, wherein, in case of the length of the long cell-code being three or more than three multiple of 16, the decomposed result matrix H is: ${H = \begin{bmatrix} H_{1} & H_{2} & \quad & \quad & \quad & \quad & \quad & \quad & \quad \\ \quad & H_{3} & H_{4} & \quad & \quad & \quad & \quad & \quad & \quad \\ \quad & \quad & H_{5} & H_{6} & \quad & \quad & \quad & \quad & \quad \\ \quad & \quad & \quad & \quad & \quad & \quad & \quad & \quad & \quad \\ \quad & \quad & \quad & \quad & \quad & H_{41} & \quad & H_{42} & \quad \\ \quad & \quad & \quad & \quad & \quad & \quad & H_{43} & \quad & H_{44} \end{bmatrix}};$ where H₁, H₃, H₅, . . . , H₄₃ are 16×16 triangle matrixes, and H₂, H₄, H₆ . . . H₄₄ are 16×16 matrixes, all other elements of the H matrix are zero.
 5. The method according to the claim 3, wherein matched filtering the received signals of all antenna units in Step E comprises, calculating the received signal of all antenna units except the midamble code through formula: e _(MF)=A×e, where e _(MF) is the received signal of all antenna units except the midamble code; e is the received signal of all antenna units, and A′ is the associate matrix of second mid-matrix. 